This invention relates to total joint structure replacement and particularly to an apparatus and method for installing a prosthetic socket structure during complete ball-and-socket type joint replacement surgery.
Complete joint replacement may be required for ball-and-socket type joints when both the anatomic ball and the anatomic socket components of the joint structure have severely degraded or otherwise been damaged. The surgical replacement procedure, called arthroplasty, includes replacing both the natural anatomic ball or head of the joint and the natural anatomic socket structure with prosthetic components. The prosthetic ball structure includes a head adapted to replace the ball structure of the natural joint and the prosthetic socket structure provides a socket implant in which the prosthetic head articulates.
Complete hip joint replacement, for example, includes removing the anatomic ball structure by resecting the femoral head and neck of the femur and then securing the replacement prosthetic ball implant to the femur with a femoral stem prosthesis that is impacted deep into the proximal and intermedular canal of the femur. Replacing the socket structure includes removing the cartilage that lines the natural acetabulum or socket and perhaps some bone to leave a substantially hemispherical shaped opening or acetabular cavity in the pelvic bone for receiving the prosthetic socket implant. After producing the desired hemispherically shaped opening, the surgical procedure includes affixing the prosthetic socket structure in the opening by either pressing, impacting, or cementing the prosthetic implant into the acetabular cavity.
There are two general prosthetic socket implant types that replace the natural acetabulum of the hip joint. The first type is a one-piece implant structure constructed of polyethylene or other high grade plastic. The one-piece implant structure includes a socket for receiving the prosthetic ball and a generally hemispherical outer surface adapted to correspond to the opening made in the pelvic bone. Installing the one-piece socket design includes cementing the outer surface of the implant to the opening made in the pelvic bone.
The second type of acetabular socket prosthesis is a two piece structure. The first piece comprises a substantially hemispherical or cylindrical acetabular shell, typically constructed of a biologically compatible metal. The second piece comprises an insert that permanently attaches substantially within the acetabular shell and forms a prosthetic socket for receiving the prosthetic ball. The insert is typically constructed of an ultra high molecular weight polyethylene or other suitable plastic. Installing the two-piece prosthetic socket structure includes either pressing or impacting the acetabular shell into the pelvic bone, commonly referred to as a "press fit," or cementing the shell into the opening with polymethelmethacricate "PMMA" or other suitable bonding compound.
In the first step of installing the two-piece socket structure, the surgeon reams out the pelvic opening until cortical bone is exposed. The reamed acetabulum is then gaged to determine the proper size implant. In either the press fit or cementing procedure, surgeons use different sized trial shells to check the size of the opening. The trial shells are identical in shape to the actual shell to be installed but are smooth on their outer surfaces so as not to grip the bone and are larger or smaller than the shell to be installed depending upon the fixation technique to be employed. For receiving a press fit acetabular shell, the surgeon selects the prosthesis so that it is somewhat larger than the inner surface of the reamed acetabulum. When the surgeon presses or impacts the shell into place, the bone compresses tightly against the outer surface of the prosthetic shell to hold the shell in place. In the case of a cemented installation, the surgeon sizes the prosthetic implant somewhat smaller than the outer surface of the acetabular shell. The gap between the shell and the bone provides space for bone cement that bonds the shell to the bone.
The prosthetic socket must align properly in the pelvic opening to provide proper prosthetic function. The acetabular component must be aligned both axially and angularly within the pelvic bone. The axial alignment relates the angle of the centerline of the prosthesis with the pelvis. The angular alignment relates the prosthesis angular rotation with respect to the superior region of the natural acetabulum. Axial alignment of the acetabular prosthesis involves the commonly referenced angles of anteversion and abduction. Typically, the axial alignment should be within 5 degrees of the ideal axial alignment to provide satisfactory results. Surgeons often align the shells based upon experiences from past operations, but, less experienced surgeons may require a guide to correctly align the shell.
Most acetabular shells include holes through which bone screws may be secured into the pubis, ischium, and superior region of the pelvis. The bone screws provide immediate fixation for the acetabular shell in the prepared pelvic bone opening and reduce the possibility of prosthetic shifting or migration. The screw holes in the acetabular shell are angularly aligned with the pubis, ischium, and superior region of the pelvis. If the acetabular shell is improperly aligned either angularly or axially, one or more of the screws could accidently extend completely through the pelvis and puncture an organ, requiring emergency surgery to repair the damage.
Prior devices for aligning and seating acetabular shells include holding devices, impacting devices, and combination holding and impacting devices. Holding devices attach to the prosthetic socket shells and allow surgeons to align the shells in the prepared opening. One device, the acetabular shell holder by Intermedics, Inc., grasped the rim of the shell but disengaged from the shell if excessively torqued. Therefore, the device could be used only to align the shell, and, because the handle of the device extended to the side of the tool, the device could not be used at all for any type of impaction. Further, because this device had to be used in conjunction with an impacting device during press-fit installations, the impacting process required a surgical assistant. Maintaining correct alignment during the impacting process was difficult due to the weak grip the instrument had on the rim of the shell.
One impacting device, the acetabular dome impactor by Biopro, included a round end that loosely engages the prosthetic shell and a handle that provided no axial alignment reference. However, because it did not firmly attach to the shell, the device provided no axial aligning force and no angular aligning support. Further, because the device was not fixed during the impacting process, misalignment during the impacting process was likely. Several other devices performed only the impacting function. These devices did not attach to the shell and typically included a shaft with one end for receiving an impact and another end for delivering the impact to the shell. With these devices, misalignment during the impacting process was likely.
The cup positioning impactor device by Ostieonics could be used to perform both the shell holding and impacting functions. This device loosely engaged a set of holes on the face of the insert and shell assembly and, could itself be used to impact the shell. Although this device did provide some angular support for aligning the shell, it provided no positive engaging means for axially aligning the shell.
Similarly, the acetabular cup positioner device by Johnson & Johnson screwed into a hole at the extreme axial center of the shell. With this device, rebound during the impacting process tended to pull the shell out of the opening. Additionally, when the device was unscrewed from the shell, the shell could twist or tilt in the opening and misalign.